Affinity based bio-sensors use an affinity probe which specifically binds to a target molecule. The target molecule is the molecule which comprises or consists of the analyte molecule that needs to be quantified. In affinity based bio-sensors the binding event is detected by a transduction scheme. This can be a direct detection of the binding event (e.g. in a label free sensor), or it may be an indirect chain of interactions that can be detected. In the last case a second probe with a label allows to obtain e.g. an optical or electrochemical signal representative for the amount of analyte being present. The label may for example be a fluorophore or an enzyme that catalyzes still another reaction that for example provides an optical or electrochemical signal.
In affinity based bio-sensors with fluorescence signal transduction, a bulk sample is sent over a surface, and the target molecule (analyte) is bound by an affinity probe on the surface. The presence of target molecules on the surface is thereby correlated with the presence of labels on the surface. These labels may for example be fluorescent. In such bio-sensors the fluorescent light is captured and its intensity is a measure for the amount of analyte molecules present in the original bulk sample. The affinity probe may for example be an antibody, an antigen, an aptamer, complementary DNA or a molecularly imprinted surface. In the affinity based fluorescent bio-sensor with evanescent excitation fluorescence, total internal reflection may be used to create an evanescent field very close to the sensor surface, to excite only those fluorophores which are present very close to the surface, and of a large fraction of which can be expected that they are bound to an affinity probe on the surface.
Despite the already existing affinity based sensors there is still room for building improved affinity based sensors.